1. The Field of the Invention
The present invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to buffer layers for preventing diffusion in a spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. The Relevant Art
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads carrying read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are now the most common type of read sensors. This is largely due to the capability of MR heads of reading data on a disk of a greater linear density than that which the previously used thin film inductive heads are capable of. An MR sensor detects a magnetic field through a change in resistance in its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic maonetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic to layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fexe2x80x94Mn) layer.
The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
FIG. 1 shows a typical prior art SV sensor 100 comprising a pair of end regions 104 and 106 separated by a central region 102. The central region 102 is formed by a suitable method such as sputtering and has defined end regions that are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an anti ferromagnetic (AFM) layer 121.
The free layer 110, spacer layer 115, pinned layer 120 and AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. A sensing device 170 connected to the leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by an external magnetic field (e.g., a field generated by a data bit stored on a disk). IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
Another type of spin valve sensor more recently developed is an anti-parallel (AP)-pinned spin valve sensor. FIG. 2 shows one representative AP-pinned SV sensor 200. The AP-pinned SV sensor 200 has a pair of end regions 202 and 204 separated from each other by a central region 206. The AP-pinned SV sensor 200 is also shown comprising a Nixe2x80x94Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208 which is made of NiO.
The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 of cobalt and a second ferromagnetic layer 216 of cobalt separated from each other by a ruthenium (Ru) antiparallel coupling layer 214. The AFM layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. A pair of hard bias layers 235 and 240, formed in the end regions 202 and 204, provide longitudinal biasing for the free layer 225.
A pair of electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the SV sensor 200. In the depicted example, the magnetization direction of the free layer 225 is set parallel to the air bearing surface (ABS) in the absence of an external field. The magnetization directions of the pinned layers 212 and 214, respectively, are also set to be perpendicular to the ABS. The magnetization directions of the pinned layers are shown as coming out of the Figure at 260 and going in at 255. The magnetization of the free layer 225 is shown set to be parallel to the ABS.
The disk drive industry has been engaged in an ongoing effort to increase the overall sensitivity, or GMR coefficient, of the SV sensors in order to permit the drive head to read smaller changes in magnetic flux. Higher GMR coefficients enable the storage of more bits of information on any given disk surface and ultimately provide for higher capacity disk drives without increased size or complexity of the disk drives. The GMR coefficient of an SV sensor is xcex94R/R, or the change in resistance of the sensor material, divided by the overall resistance of the material when the sensor material is subjected to a changing magnetic field. The GMR coefficient is dependent on both the xe2x80x9csoftnessxe2x80x9d of the material and its overall resistance.
A change in resistance of the sensor material can be easily measured only if the change is large compared to the overall resistance R of the material. Thus, a low overall resistance R, combined with a high change in magnetoresistance, xcex94R, will produce a high GMR coefficient.
Other properties relevant to the performance of a GMR head include magnetostriction, exchange coupling between the AFM and the pinned layer or layers, and the electrical resistivity of the AFM. Magnetostriction is a measure of the stress or deformation of a material when it undergoes a change in magnetism. It is desired in the construction of spin valves to keep magnetostriction to a minimum because deformation of the GMR head materials can cause poor interfacing between layers and nonlinear performance as magnetic flux changes.
Exchange coupling between the AFM and the pinned layers is important because magnetic flux from the AFM must reach the pinned layer with a minimum of reluctance or leakage in order to keep the magnetic moment of the pinned layer at a consistent orientation. An inadequate exchange coupling may cause poor pinning, thereby reducing the sensitivity of the GMR head.
It is also vital that the current through the spin valve sensor be confined to the pinned and sensing portions of the spin valve sensor. If current is permitted to shunt through the AFM layer, the magnetoresistance recorded by the sensor will be artificially low, thus producing a lower GMR coefficient and a nonlinear signal. Thus, the material selected for the AFM layer must possess a high electrical resistivity in order to prevent shunting.
Prior art drive heads have been produced by forming a seed or buffer layer on or near the substrate, and then forming the remaining layers on top of the seed layer. The crystalline structure and orientation of the seed layer determine the configuration of the remaining layers. Materials such as NiFe have previously been used to form all or part of the seed layer.
Recently, magnesium alloys such as PtMn have been used to form the AFM layer. The Mn alloys exhibit strong exchange coupling properties and allow for thinner AFM layers to be used. This provides greater freedom for increasing the number or thickness of other layers without substantially increasing the area of the spin valve sensor.
Nevertheless, when spin valve sensors are formed of materials such as PtMn, annealing at high temperatures is required. This annealing process creates other problems that can affect the performance of the spin valve sensor. These problems include degradation of the pinning field, intermixing of the elements that comprise the pinned and free layers, and an undesirably low resistance in non-sensing layers. The prior art has attempted to reduce the degradation of the pinning field as a function of temperature by using a PtMn AFM/NiFe free layer as opposed to a PtMn AFM/Co free layer. Nevertheless, even in sensors with this composition, some degradation continues to occur.
From the above discussion, it can be seen that it would be beneficial to further improve the GMR coefficient of current spin valve sensors by providing a more substantial decrease in the degradation of the pinning field during annealing and raising the resistivity to the shunt current that flows between the pinned layer and the free layer.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available spin valve sensors. Accordingly, it is an overall object of the present invention to provide an improved spin valve sensor that overcomes many or all of the above-discussed shortcomings in the art.
To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiment, an improved spin valve sensor is provided. The spin valve sensor of the present invention in one embodiment comprises a buffer layer positioned between a pinned layer of ferromagnetic material and an antiferromangentic (AFM) layer. The AFM layer may be formed of a Mn alloy such as PtMn. The buffer layer is preferably formed of an NiFeX alloy where X is an element selected to decrease the degradation of the pinning field as a function of temperature, decrease the intermixing of the materials in the AFM and pinned layers and to act as a barrier to shunt currents flowing between the AFM and pinned layers. The element X in one embodiment is selected from the group consisting of Nb, Mo, Cr, and Ta.
The spin valve sensor may comprise a cap layer, a free layer, a pinned layer of ferromagnetic material as discussed above, an antiferromagnetic (AFM) layer, a substrate, and a seed layer. In one embodiment, the AFM layer is formed of an alloy containing Mn and the pinned layer(s) are formed of Co or CoFe. Nevertheless, the buffer layer of the present invention is intended for use with any type of spin valve sensor having a construction favorable to the use of a buffer layer that prevents diffusion during annealing.
The spin valve sensor of the present invention may be incorporated within a disk drive system comprising a magnetic recording disk; an anti-parallel (AP)-pinned spin valve (SV) sensor configured in the manner discussed above; an actuator for moving the spin valve sensor across the magnetic recording disk so the spin valve sensor may access different regions of magnetically recorded data on the magnetic recording disk; and a detector. The detector may be electrically coupled to the spin valve sensor for detecting changes in resistance of the sensor caused by rotation of the magnetization axis of the free ferromagnetic layer relative to the fixed magnetizations of the AP-pinned layer in response to magnetic fields from the magnetically recorded data.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.